1. Field of the Invention
This invention relates to self-adjusting pliers having jaws which pivot about a jaw portion first pivot point during a first phase and a jaw portion second pivot point during the second closing phase and, more specifically, to a pair of pliers which utilizes a cam during the transition between the first phase of operation and the second phase of operation.
2. Background Information
Pliers are used to grasp or otherwise act upon a workpiece. A pair of pliers includes two elongated members joined by a pivot pin at an intersection on the medial portion of the members. One end of each elongated member forms a jaw and the other end of the elongated members forms a handle. A workpiece located between the jaws may be grasped by an operator drawing the handle portions towards each other. Hand tools have also been constructed using two elongated members which do not intersect, but which are joined by a link member, see, e.g., U.S. Pat. No. 3,600,986. The link member forms a toggle joint with one of the handle portions. The toggle joint augments to force applied by the user as the angle of toggle joint approaches 180 degrees.
The basic pair of pliers was improved to be self-adjusting by incorporating a slot in one elongated member which allows the jaw portion of the other elongated member to be moved relative to the pivot point, see, e.g., U.S. Pat. No. 1,508,510. Further improvements to pliers allow the pliers to self-adjust to the size of the workpiece and further incorporate a two pivot system, see, e.g. U.S. Pat. No. 6,014,917. The two pivot system provides for a first pivot point that allows the jaw members to close until contacting a workpiece whereupon the pivot point shifts to the second pivot point, which is typically located closer to the workpiece, thereby giving the user greater leverage while drawing the handle portions towards each other. The second pivot point is typically created by a pawl which engages a toothed rack. During the initial phase of operation, the pawl is spaced apart from the rack, or allowed to slide over the rack. When the jaws of the pliers contact the workpiece, the pawl is shifted into engagement with the rack thereby creating the second pivot point.
The disadvantage of pliers which utilize a pawl to form the second pivot point is that such pliers are subject to a periodic force variation as the pawl engages the rack. That is, because the teeth on the rack are disposed at fixed incremental locations and because the pawl moves over the rack teeth, the pawl may not be positioned to engage a tooth on the rack at the point when the jaws of the pliers contact the workpiece. When this occurs, the pawl must move backwards on the rack to engage a tooth. The larger the tooth, the more variation there is between the point at which the jaws contact the workpiece and the point at which the second pivot is created. One method of reducing the variation is to utilize racks having smaller teeth. Smaller teeth, however, are weaker and may not be able to tolerate the stress placed on the tooth. Additionally, fine teeth are difficult to manufacture and are subject to degradation from repeated use.
Further, prior art pliers have not successfully combined the self-adjusting and force augmenting features with a locking feature. For example, a typical toggle joint tool utilizes an over-toggle pin to lock the tool. An over-toggle pin, however, requires that the toggle joint pass through the toggle position to engage. After the toggle joint passes through the toggle position, the closing force is reduced.
There is, therefore, a need for a pair of pliers that operate about a first pivot during a first pivot phase and about a second pivot during a second pivot phase and which does not utilize a pawl and tooth rack combination to create the second pivot.
There is a further need for a pair of pliers that operate about a first pivot and a second pivot which does not have a variation during the transition between the first pivot and the second pivot.
There is a further need for a pair of pliers that incorporates a self-adjusting mechanism, a force augmentation mechanism and a locking mechanism.